Question

已知x+y=3,xy=-7,求:

① x²+y² ② x²-xy+y² ③ (x-y)² 不会做啊，求解答，O(∩_∩)O谢谢

Answer 1

1. x²+y²=（x+y)^2-2xy

2. x^2+y^2-xy=（x+y)^2-3xy

3. (x-y)^2=(x+y)^2-4xy

   然后分别代入即可。

   
   

Answer 2

解①x^2+y^2
=(x+y)^2-2xy
=3^2-2(-7)
=9+14
=23
②x^2-xy+y^2
=x^2+y^2-xy
=23-(-7)
=30
③（x-y）^2
=x^2-2xy+y^2
=x^2+y^2-2xy
=23-2（-7）
=37

Answer 3

x+y=3,
(x+y)^2=9
x²+y² +2xy=9

xy=-7
①x²+y² =9+14=23

② x²-xy+y²
=23+7

=30

③ (x-y)²
=x²-2xy+y²

=23+14
=37

Answer 4

Answer 5